1. Field of the Invention
The invention relates to an image forming apparatus, such as a copy machine, a facsimile machine, or a printer.
2. Description of the Related Art
This type of image forming apparatus is described in Japanese Patent Application Laid-open No. 8-83006. This image forming apparatus forms a monochrome image on a recording sheet by a combination of a photosensitive element serving as a latent image carrier and a transfer roller serving as a nip forming member, which comes into contact with the photosensitive element to form a transfer nip. A transfer bias having a polarity opposite to the normally charged polarity of toner is applied to the transfer roller. A toner image on the photosensitive element is transferred to a recording sheet serving as a recording member fed into the transfer nip. On the surface of the photosensitive element, all of a non-image portion and an image portion (latent image portion) are charged with the same polarity with the normally charged polarity of toner, and the potential of the non-image portion becomes higher than the potential of the image portion. At the exit of the transfer nip, a current flows between the nip forming member and the photosensitive element in accordance with separating discharge therebetween. At this time, a larger current flows in the non-image portion having a higher potential than the image portion. For this reason, when the image area ratio on the photosensitive element at the exit of the transfer nip is comparatively low, if a larger current is not output from a power supply compared to a case where the image area ratio is relatively high, a necessary current may not flow in the image portion, causing defective transfer. If defective transfer occurs, irregularity in image density depending on the image area ratio occurs. Thus, this image forming apparatus is configured such that the output target value of the transfer current from the power supply differs depending on the above-described image area ratio. With this configuration, it is possible to obtain stable image density regardless of the image area ratio.
Meanwhile, in the related art, an image forming apparatus is known in which a color image is formed through so-called superimposing transfer (for example, see Japanese Patent Application Laid-open No. 2003-186284). Superimposition transfer is processing in which a plurality of toner images on a latent image carrier, such as a photosensitive element, are superimposed on a transfer member, such as an intermediate transfer member. As a method which realizes superimposing transfer, various methods are known.
For example, in the image forming apparatus described in Japanese Patent Application Laid-open No. 2003-186284, superimposing transfer is realized by a so-called tandem method. Specifically, the image forming apparatus has four photosensitive elements which respectively form toner images of Y (yellow), M (magenta), C (cyan), and Bk (black). The image forming apparatus also has an intermediate transfer belt serving as a nip forming member which comes into contact with the photosensitive elements to form Y, M, C, and Bk primary transfer nips. At the Y primary transfer nip, the Y toner image on the Y photosensitive element is transferred to the intermediate transfer belt. Thereafter, in the M primary transfer nip, the M toner image on the M photosensitive element is transferred to be superimposed on the Y toner image of the intermediate transfer belt. Hereinafter, similarly, in the C and Bk primary transfer nips, the C and Bk toner images are transferred to be superimposed on the Y and M toner images of the intermediate transfer belt. With superimposing transfer, it is possible to form a color image on the intermediate transfer belt.
An image forming apparatus is also known in which superimposing transfer is realized using a photosensitive element, four developing units developing a latent image formed on the photosensitive element with Y, M, C, and Bk toners, respectively, and an intermediate transfer belt. In this type of image forming apparatus, while the intermediate transfer belt is moving substantially over four revolutions, toner of a different color is formed on the photosensitive element in each revolution and transferred to the intermediate transfer belt in a superimposing manner. In this way, it is possible to form a color image on the intermediate transfer belt.
With the configuration in which a color image is formed by the transfer method in each revolution or the above-described tandem method, similarly to the image forming apparatus described in Japanese Patent Application Laid-open No. 8-83006, irregularity in image density depending on the image area ratio may occur. Thus, the inventors have conducted an experiment in which, in a tandem-type color printer tester, the target value of an output current from each of the Y, M, C, and Bk primary transfer power supplies differs depending on the image area ratio of a corresponding one of the Y, M, C, and Bk photosensitive elements. When this happens, the toner images of the respective colors can be primarily transferred efficiently from the photosensitive element to the intermediate transfer belt, but a toner image on the belt is reversely transferred noticeably to the non-image portion of the photosensitive element in a downstream-side primary transfer nip. For example, the Y toner image which has been satisfactorily transferred to the intermediate transfer belt in the Y primary transfer nip is reversely transferred in large quantity to the non-image portions of the M, C, and Bk photosensitive elements in the downstream-side M, C, and Bk primary transfer nips. Similarly, the M toner image is reversely transferred in the C and Bk primary transfer nips, and the C toner image is reversely transferred in the Bk primary transfer nip.
Reverse transfer easily occurs in a state where an area having a comparatively low image area ratio of the circumferential surface of the photosensitive element and a toner image which has already been transferred to the intermediate transfer belt are moved simultaneously into a primary transfer nip. As in the related art, with the configuration in which a constant transfer current is output regardless of the image area ratio of the photosensitive element, if such a state is reached, most of the photosensitive element within the nip becomes a non-image portion. For this reason, a current easily flows between the photosensitive element and the belt, reducing the potential of the belt. Thus, the potential difference between the non-image portion of the photosensitive element and the intermediate transfer belt decreases, such that discharge does not easily occur between the non-image portion of the photosensitive element and the intermediate transfer belt. As a result, reverse transfer of toner is suppressed. Meanwhile, in the color printer tester in which the target value of the transfer current varies depending on the image area ratio, if the above-described state is reached, the target value increases so as to allow a sufficient current to flow in the image portion having a small area, such that the potential of the belt is scarcely reduced. It could be seen that, when this happens, discharge is not suppressed between the non-image portion of the photosensitive element and the belt, such that reverse transfer of toner noticeably occurs.
Thus, the inventors have carefully studied an appropriate set range of the target value of the transfer current and have understood the following. That is, the above-described color printer tester is configured such that the toner images are sequentially transferred to the belt in order of Y, M, C, and Bk. While the toner image which has been transferred to the intermediate transfer belt in the uppermost stream-side Y primary transfer nip sequentially passes through the M, C, and Bk primary transfer nips, it is impossible to completely eliminate loss of toner in the toner image. In any case, a small amount of toner is stuck to the photosensitive element in the primary transfer nip for the second and latter colors. For this reason, the Y toner image which passes through the primary transfer nips for all colors is likely to decrease in image density compared to the M, C, and Bk toner images which pass through only a portion of the primary transfer nips. Thus, in the Y primary transfer nip, it is preferable to set the target value of the transfer current such that the maximum transfer efficiency is substantially obtained. Specifically, in the primary transfer nip, as the primary transfer current increases, the transfer efficiency tends to increase. However, if the primary transfer current excessively increases, the transfer efficiency is significantly deteriorated adversely. This is because the potential difference between the image portion of the photosensitive element and the belt increases more than the discharge start voltage, such that toner on the image portion is reversely transferred due to discharge therebetween. If the potential difference between the image portion of the photosensitive element and the intermediate transfer belt is maintained at a value slightly smaller than the discharge start voltage, it is possible to obtain substantially the maximum transfer efficiency. The transfer current value which maintains the above-described potential difference at a value slightly smaller than the discharge start voltage differs depending on the image area ratio on the photosensitive element. It should suffice that the relationship between the transfer current value and the discharge start voltage is found in advance and stored as an algorithm (relationship expression, data table, or the like). Meanwhile, if the above-described potential difference is set at a value slightly smaller than the discharge start voltage, the potential difference between the non-image portion of the photosensitive element and the intermediate transfer belt increases more than the discharge start voltage. This is because the non-image portion and the image portion both have a polarity opposite to the transfer bias, and the potential of the non-image portion is higher than that of the image portion. For this reason, in the M, C, and Bk primary transfer nips, if the target value of the transfer current is set in the same manner as the Y primary transfer nip, noticeable reverse transfer occurs.